Fixing Metal back box to a steel box column

[Spinny]

So I am looking to position a metal back box for a double socket just in front of a steel box column. Not sure of the thickness of the steel - I guess maybe 8mm or 6mm.

As it is a steel column rising from foundation level it is or feels cold to the touch, so I plan to insert a layer of insulation e.g. 10mm XPS board between the back box and the steel to heat insulate the back box.

What size Techscrew should I use to screw the back box into the steel column ?

How do I insulate the screw to stop it cold bridging the XPS insulation to make the back box cold ?

 

[Temp]

Id probably glue it in place.

[Nickfromwales]

Yup. 
 

CT1 to hold both the XPS to the steel and the box to the XPS.
 

Don’t use bare XPS as it’s quite friable, get something like Jackoboard or tile backer board, the type with the grey gritty surface coating to accent adhesives, and that’ll hold up much better. 
 

I’d use a pvc conduit back box here, and defo not a metal one here. You can still bury that and plaster to the edges of it, so zero chance of cold getting any further from the steel than you want. The conduit boxes aren’t as fragile as the regular surface mountable back boxes, as they’re made from a different, softer plastic. Link  

 

Tbh the entire rising faces of this steel should be clad with the insulation material, not just where the socket box is.

[saveasteading]

8 hours ago, Spinny said:

How do I insulate the screw

You don't worry about it. The heat loss will be miniscule.

For perspective, you could  counter it by closing the front door a second more quickly or turning a light off for 10 seconds... I'm guessing.

 

Depending how much space you have, it makes it easier to screw or glue a timber or ply plate to the steel, and then the box fitting is by wood screws.

[JohnMo]

8 hours ago, Spinny said:

How do I insulate the screw

I would be more concerned that you actually discovered the steel and you can put your hand on it. It should be fully insulated not hidden behind the plasterboard uninsulated.

[Spinny]

Ta for comments.

Steel is not boarded over yet. So yes will be insulating between it and the p/board. It supports an RSJ across the inner leaf above my bifold opening. So in theory it sits in the warm zone inside my wall insulation, and underneath my warm roof. But of course it has cold rising up the column from foundation level and nobody thought about insulating pads and stuff when it went in, so an unfortunate vertical thermal bridge I suppose.

 

Have a light switch to go in too and will be using a 47mm box to accomodate a smart home relay module, probably got about 60mm between steel and back of P/B.

 

Can you get surface pattress boxes with an adjustable lug ? (Just in case my spirit level isn't working that day.)